The invention relates to a low-pressure discharge lamp provided with a tubular discharge vessel having an axis and having a wall which encloses a discharge space containing an ionizable filling in a gastight manner, while a pair of electrodes are arranged in the discharge space and the wall of the discharge vessel is provided with an optically active layer of a material at an inner surface, which optically active layer is interrupted by an elongate window extending in the direction of the axis.
The invention also relates to a method of manufacturing such a low-pressure discharge lamp wherein the material of the optically active layer is provided on said inner surface, after which the material situated within the window to be formed is removed.
In the present description and claims, the term "optically active layer" is understood to mean a layer on or in which an interaction with the radiation originating from the discharge space takes place. It is achieved by means of the window in the optically active layer that the radiation emitted by the lamp through the window has a comparatively high intensity compared with the intensity of the radiation of lamps having an optically active layer extending over the entire inner surface. Such lamps are highly suitable for applications where a comparatively high intensity of the light source is required, such as in an LCD unit or applications where the radiation originating from the lamp must be concentrated into a line-shaped region, such as in a device for the automatic reading of documents.
A method of the kind mentioned in the opening section by which such lamps can be manufactured is known from EP 0 464 723 A2. In the known method, a scraping member is moved through the discharge vessel and pressed against the wall by means of a magnetic field. In this manner a window is exposed within the optically active layer in the lamp, in this case a luminescent layer and/or a reflecting layer.
A disadvantage of the known method is that the contact between the lamp and the tool by which the window is provided may easily lead to defects of the lamp. The scraping member, for example, may introduce impurities into the discharge vessel. Moreover, the scraping member is subject to wear because it is in contact with the discharge vessel while moving. Wear of the scraping member may result in damage to the discharge vessel wall and/or an imperfect removal of material within the window to be formed.
In the publication cited above, the use of the method is described for lamps whose discharge vessels have an internal diameter of between approximately 3.75 and 5.25 min. In proportion as the discharge vessel diameter, and thus the maximum admissible cross-section of the scraping member decreases, the magnetic field exerts a smaller force on the scraping member. The effectiveness with which the material within the window is removed is reduced thereby. During scraping away of material for forming a window, the material scraped away is usually removed by means of a gas, for example air, which is made to flow through the discharge vessel in order to render possible an unhampered movement of the scraping member. As the discharge vessel diameter is smaller, however, there will be less space between the scraping member and the discharge vessel available for allowing the gas to pass. This may result in an insufficient removal of the material from the discharge vessel, so that it hampers the movement of the scraping member.